1. Field of the Invention
This invention generally relates to a magnetic positioning device for use in positioning an object in various machines such as machine tools and industrial robots, and, in particular, to a linear pulse motor for positioning an object at high accuracy. This invention also relates to a linear motion unit including such a linear pulse motor and a guide mechanism associated therewith.
2. Description of the Prior Art
A linear pulse motor or linear motion unit for use in positioning an object along a straight line is well known in the art and a typical prior art linear pulse motor is illustrated in FIG. 11. The motor shown in FIG. 11 is of the two phase excitation type and it includes a primary side unit 41 for supplying a displacing pulse signal and a secondary side unit 42 for generating a thrust in cooperation with the primary side unit 41. The primary side unit 41 includes a permanent magnet 43 formed in the shape of a rectangular plate and a pair of electromagnets 44 and 45 coupled to the bottom surface of permanent magnet 43 at opposite ends thereof. The electromagnets 44 and 45 include iron cores whose pole portions 44a, 44b and 45a, 45b extend downwardly, whereby the bottom end portions of these pole portions serve as inductor teeth. Coils 44c and 45c are wound around the respective iron cores. A back plate 47 of a material having a high permeability is fixedly attached to the top surface of the permanent magnet 44. A plurality of threaded holes 48 are provided extending through the back plate 47 and the permanent magnet 43, and bolts (not shown) may be threaded into these holes 48 to have the primary side unit 41 fixedly mounted on a table 50.
Regarding the secondary side unit 42, it is generally in the shape of an elongated, rectangular plate having a high permeability and provided with a number of inductor teeth 42a arranged at a predetermined pitch .tau..sub.2, each extending in the transverse direction with respect to the longitudinal axis of the secondary side unit 42. The secondary side unit 42 is typically fixedly mounted on a bed 51 of a machine tool or the like, for example, by means of bolts (not shown).
The primary and secondary side units 41 and 42 are arranged such that their respective inductor teeth are in an opposed and spaced-apart relationship. Although not shown specifically in the drawings, there is typically provided a guide means for guiding the primary side unit 41 relative to the secondary side unit 42 with a predetermined gap therebetween (indicated by e.sub.2 in FIG. 11) such that a relative linear motion may be produced between the primary and secondary side units 41 and 42. A linear motion unit may be created by combining such a guide means with the above-described linear pulse motor.
In the above-described example, a single linear pulse motor is mounted on the bed 51 with its primary side unit 41 carrying the table 50. With this structure, an object (not shown) may be carried on the table 50 as fixedly attached thereto for positioning at a desired location since the table 50 may move back and forth in either direction as indicated by the double arrow F. Alternatively, two or more such linear pulse motors may be provided in a line or in a side-by-side relationship, if desired.
Now, also referring to FIGS. 12 through 15, a description will be had with respect to the operation of a linear motion unit including a linear pulse motor having the above-described structure. Since the principle of operation of the linear pulse motor itself is well known in the art, its detailed description will not be repeated here. It is to be noted that reference characters 52 through 59 have been used in FIGS. 12 through 15 just for the sake of convenience and they are all part of the inductor teeth 42a of the secondary side unit 42. In addition, in FIGS. 12 through 15, reference characters S and N indicated above and below the permanent magnet 43, respectively, indicate the magnetic polarities of the permanent magnet 43.
As a brief summary of the principle of operation of a linear pulse motor of the above-described permanent magnet type (or PM type), the magnetic interactive force between the opposed teeth is determined by a combination of a magnetic field produced by the permanent magnet and another magnetic field produced by the electromagnets and the table 50 takes a position where these magnetic interactive forces are balanced. When the energization state of the electromagnets is varied, the magnetic field produced by the electromagnet changes, which, in turn, changes the magnetic interactive forces to thereby apply a thrust to the table 50.
That is, assuming that FIG. 12 illustrates the initial balanced condition with current flowing through coils 44c and 45c of the electromagnets 44 and 45 of the primary side unit 41 as indicated by the arrows, respectively, a strong magnetic flux .PHI..sub.2, which is a combination between a magnetic flux due to the permanent magnet 43 and another magnetic flux due to each of the coils 44c and 45c, is present between each of the magnetic pole portions 44a and 45b and a corresponding one of the inductor teeth 52 and 57 of the secondary side unit 42. On the other hand, a weak magnetic flux is present between each of the remaining magnetic pole portions 44b and 45a of these electromagnets 44 and 45 and an adjacent inductor tooth of the secondary side unit 42 since a subtractive relationship is present between the magnetic flux due to the permanent magnet 43 and the magnetic fluxes due to the coils 44c and 45c there. It may be so set that the magnetic flux due to the permanent magnet 43 and each of the magnetic fluxes due to the coils 44c and 45c cancel each other.
Now, let us assume that the current flowing through the coil 44c of electromagnet 44 has been reversed and thus the magnetic polarities of electromagnet 44 have been reversed as shown in FIG. 13. Then, a strong magnetic flux .PHI..sub.2 is now present between the magnetic pole portion 44b of electromagnet 44 and an adjacent inductor tooth 54 of secondary side unit 42 while the magnetic flux .PHI..sub.2 which existed between the magnetic pole portion 44a and the inductor tooth 54 in the initial condition has disappeared. As a result, an attractive force between the magnetic pole portion 44b, together with an attractive force due to the strong magnetic flux .PHI..sub.2 present between the magnetic pole portion 45b of electromagnet 45 and the inductor tooth 57, causes the primary side unit 41, and thus table 50, to move over a distance corresponding to 1/4 of pitch .tau..sub.2 of inductor teeth.
Then, as shown in FIG. 14, the current flowing in the coil 45c of electromagnet 45 has been reversed in direction so that the magnetic polarities of electromagnet 45 has been reversed. Thus, a strong magnetic flux .PHI..sub.2 is now present between the magnetic pole portion 45a of electromagnet 45 and its adjacent inductor tooth 56 of secondary side unit 42 and the magnetic flux which was present between the other magnetic pole portion 45b and the inductor tooth 57 disappears. Therefore, an attractive force is produced between each of the magnetic pole portions 44b and 45a and a corresponding one of the inductor teeth 54 and 56 so that the table 50 is again caused to move over a distance corresponding to 1/4 of pitch .tau..sub.2.
Then, as shown in FIG. 15, the direction of the current flowing in the coil 44c of electromagnet 44 is again reversed so that a strong magnetic flux .PHI..sub.2 is now present between the magnetic pole portion 44a of electromagnet 44 and its adjacent inductor tooth 53 of secondary side unit 42; on the other hand, the strong magnetic flux which existed between the other magnetic pole portion 44b and its adjacent inductor tooth 54 disappears. As a result, an attractive force is now effective between each of the magnetic pole portions 44a and 45a and a corresponding one of inductor teeth 53 and 56 so that the table 50 is caused to move over a distance corresponding to 1/4 of the pitch. Thereafter, the current flowing each of the coils 44c and 45c of the respective electromagnets 44 and 45 is set in the condition shown in FIG. 12, and, therefore, the table 50 is again caused to move over a distance corresponding to 1/4 of the pitch. As a result, the table 50 comes to be shifted in position over a distance corresponding to a single pitch .tau..sub.2.
In this manner, the above-described operation is repeated over a desired number of times so that the table 50 comes to be positioned at a desired location. Although the table 50 has been shifted to the right in the above-described example, the table 50, of course, may move to the left. Besides, although the primary and secondary side units 41 and 42 have been set as moving and stationary sides, respectively, in the above-described example, the primary side unit 41 may be set as a stationary side with the secondary side unit 42 as the moving side.
The resolution of the above-described linear pulse motor, i.e., the pulse pitch traversed by the primary side unit 41 per single pulse signal applied, is determined by dividing the pitch .tau..sub.2 of the inductor teeth 42a of secondary side unit 42 by the number of excitation modes of the coil. Thus, the following three approaches are conveniently and mainly adopted so as to obtain an increased resolution.
(1) Reducing pitch .tau..sub.2 by increasing the manufacturing accuracy at the primary and secondary side units; PA1 (2) Increasing the number of excitation modes of the coil by adopting a multi-phase excitation method; and PA1 (3) Adopting the so-called microstep scheme wherein the excitation electromagnetic current of the coil is subdivided to adjust the magnetic force finely, thereby finely varying the position of magnetic balanced location.
However, there is a limit in the improvement of resolution even in any of the above-described schemes were adopted, and there still remains a need to provide an improved resolution on the order of 1 micron or submicrons.